<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="style.css">
    <title>Document</title>
</head>

<body>
    <!-- 利用input的radio单选框作为主题切换的控制器 -->
    <input type="radio" name="theme" id="red" checked> 
    <input type="radio" name="theme" id="blue">
    <label for="red" class="red"></label>
    <label for="blue" class="blue"></label>
    <!-- 用于显示需要切换主题的元素 -->
    <div class="main">
        <ul>
            <li><input type="text" placeholder="请输入用户名"></li>
            <li><input type="password" placeholder="请输入密码"></li>
            <li><input type="password" placeholder="请再次输入密码"></li>
            <li><a href="#">返回</a><a href="#">确定</a></li>
        </ul>
    </div>
</body>

</html>